1. Field of the Invention
The present invention relates to aqueous ink compositions for ball-point pen and, more particularly, to such aqueous ink compositions which are superior in lubricity and hence are free of clogging at the tip of ball-point pen even after prolonged use.
2. Related Art
It has been a conventional practice to prepare an aqueous ink composition for ball-point pen by adding a water-soluble organic solvent of a polyvalent alcohol or a derivative thereof to an aqueous solution containing a water-soluble dye, water-dispersible pigment or colored resin emulsion. Attempts have been made to enhance the lubricity of the aqueous ink composition by blending the ink composition with such lubricants as an alkali metal salt of an unsaturated fatty acid, or an alkali metal salt or amine salt of an anionic surface active agent having a carboxyl group. Refer to Japanese Unexamined Patent Publications No.61-16974 (1986), No.61-174278 (1986) and No.4-164977 (1992) and Japanese Examined Patent Publication No.5-54875.
With any of the foregoing conventional lubricants, however, the resultant ink has a difficulty in simultaneously satisfying requirements for lubricity and for other writing characteristics. For instance, an ink with satisfactory lubricity often exhibits extremely decreased surface tension, which may result in practical problems such that the ink is liable to blur on paper. Further, many ink compositions show unsatisfactory lubricity due to influences of selected pigment or resin, even if lubricants are added thereto.
It is, therefore, an object of the present invention to provide an aqueous ink composition for ball-point pen which overcomes the foregoing drawbacks of the conventional ink compositions, assures long-term stability, and is less liable to blur on paper.
As a result of intensive study to solve the foregoing problems, it has been found that the problems associated with the conventional ink compositions can be solved by adding a specific compound to an aqueous ink composition for ball-point pen comprising at least an aqueous solvent composed of a water-soluble organic solvent and water, a surface active agent or water-soluble resin as a dispersant, and a coloring agent.